1. Field of the Invention
This invention relates to memory subsystems and, more particularly, to memory controller design.
2. Description of the Related Art
Computer systems include one or more processors that execute instructions from a memory medium. The memory medium is typically one or more levels of cache and a main memory. The main memory may be made up of one or more memory modules, e.g., Dual In-Line Memory Modules (DIMMs) or Fully-Buffered DIMMs (FB-DIMMs). A memory subsystem of a computer system typically includes a memory controller, which is connected to the main memory. The main function of the memory controller is to schedule and coordinate the execution of read and write requests (i.e., memory accesses) to main memory. The memory controller may include at least one queue to schedule the read and write requests. In most cases, the memory controller includes a queue for the read requests and a separate queue for the write requests.
Memory subsystems are typically configured to place memory modules that are not being used into a low power mode to save power. This is usually the case because in typical systems a large portion of the power dissipation is in the main memory. In most cases, memory accesses are typically executed in the order that they are received at the appropriate queue of the memory controller. The memory controller may receive several read commands in a row and each command may access a different memory module. For example, if the memory subsystem includes eight memory modules, the memory controller may receive eight read commands each accessing a different memory module. In this situation, since typical memory controllers execute the read commands in a row, the powers savings would be minimal if some of the memory modules entered into a low power mode because all the memory modules would be accessed within a short period of time. Since read and write requests are received randomly and are executed in the order they are received, the memory modules may be constantly switching from a normal power mode to a low power mode, which may result in minimal or no power savings in most situations. In some cases, the constant switching from the two different power modes may actually increase the power consumption in the system.